Electrically Anisotropic Crust From Three-Dimensional Magnetotelluric Modeling in the Western Junggar, NW China

Abstract

An island arc environment related to intraoceanic subduction or ridge subduction from the Late Carboniferous to the Early Permian has been proposed for the formation of Western Junggar, NW China, situated in the southwest of Central Asian Orogenic Belt. However, the details and consequences of the subduction remain controversial. To further identify the relics of the Late Carboniferous subduction at depth, three magnetotelluric profiles were deployed. Phase tensors, real induction arrows, and three-dimensional (3-D) isotropic resistivity models indicate the presence of a 3-D anisotropic resistivity structure in the crust. Consequently, a 3-D anisotropic resistivity model was constructed by forward modeling. The 3-D anisotropic resistivity model contains two azimuthally anisotropic middle-upper crust anomalies with minimum resistivity striking N90°E at depths of 5 to 20 km, and an azimuthally anisotropic lower crust anomaly with minimum resistivity striking N20°E at depths of 20 to 46 km. The electrically anisotropic anomalies are closely related to relatively high shear-wave velocities with significant negative radial anisotropy. The anisotropies in the middle-to-upper crust and the lower crust are related to the oceanic ridge (N90°E) subduction with a slab window in the Late Carboniferous and the remnant oceanic slab, respectively. The magnetotelluric observations support a NW70° subduction with an intraoceanic ridge-transform system in the Late Carboniferous and a remnant oceanic slab trapped in Western Junggar.